What Is the Glycemic Index?

The glycemic index (GI) is a relative ranking of how different carbohydrates affect blood sugar. When you have type 2 diabetes, one of the best ways to control your glucose levels is to eat foods that don't cause major blood sugar (glucose) spikes.

Knowing the glycemic index of the carbohydrates you eat can help you fine-tune your meals to keep your blood sugar within a normal range. Foods with a higher GI value are more likely to spike your blood sugar than foods with a lower GI.

This article explains the glycemic index and how it works. It also provides glycemic index charts that show low GI, moderate GI, and high GI carbohydrates.

What Is the Glycemic Index?

The GI is a rating system that ranks carbohydrates on a scale of 1 to 100 based on how much they raise blood sugar.

Processed foods such as candy, breads, cake, and cookies have a high GI, while whole foods such as unrefined grains, non-starchy vegetables, and fruits tend to have a lower GI.

Carbohydrates with a low GI value are digested, absorbed, and metabolized more slowly than their high-GI counterparts. They typically cause a lower and slower rise in blood glucose and, subsequently, insulin levels as well.

Foods that are primarily fat or protein are not included in the index because they have a minimal impact on blood sugar levels.

Glycemic Index Chart for Common Foods

The GI values can be broken down into three ranges. Food with a low GI is a food that won't raise your blood sugar as much as a food with a medium or high GI.

• Low GI: 55 or less
• Medium GI: 56 to 69
• High GI: 70 to 100

The following charts highlight low, medium, and high GI foods based on data from the American Diabetes Association.

How Glycemic Index Is Measured

Glycemic index values were developed by a rigorous testing process using 10 or more people for each food.

Researchers measured blood sugar levels of healthy volunteers before and two-hours after eating 50 grams of the same digestible carbohydrate (the test food). The points were then plotted on a graph and researchers determined the area under the curve (AUC) of their glucose response.

At a separate date, the same 10 people consumed 50 grams of pure glucose (the reference food), and researchers again measured each person's glucose response AUC two hours after consumption.

The GI value of the test food is then calculated by dividing the glucose AUC for the test food by that of the reference food for each person. The final GI value is an average of those 10 numbers.

Benefits of Referring to the Glycemic Index

Since it's the carbohydrates in food that raise blood sugar, understanding GI can help you figure out which foods are best for glucose management.

Among the benefits of following the GI list when planning your meals:

• It helps you be more mindful of your carb choices without fully restricting or severely limiting your intake.
• If you aim for a low-GI diet, you'll naturally be focusing on whole grains, fruits, vegetables, and legumes, as opposed to the higher-GI end of the spectrum, which includes more processed foods.
• Depending on your health goals, following a GI-based diet might mean you'll be able to rely less on standard dieting measures, such as calorie counting or regimented portion control.
• Simply being more mindful of your carb choices rather than severely limiting them can also be more sustainable in the long run, as compared to more restrictive diets.

Where the Glycemic Index Falls Short

Critics of the GI system note it has several flaws that can make it an unreliable measurement. GI looks strictly at the carb count. Basing a diet around GI only means you would be ignoring a lot of other helpful information to determine the true health value of a food.

The GI index doesn't take into account:

• How much food is being eaten
• Other nutrients such as protein, fat, vitamins, minerals, and antioxidants
• Other components of the meal, which can change the effect of blood sugar rise

For example, eating an apple on its own may result in a different blood glucose response than if you ate it with some peanut butter. Protein and fat can delay carbohydrate metabolism and, therefore, result in a slower blood sugar rise. 

Glycemic Index vs. Glycemic Load

To counteract some of the issues with glycemic index, researchers developed the glycemic load (GL) measurement. Unlike GI, GL accounts for the quantity of the food being eaten. The main difference between GI and GL is:

• Glycemic index is based on eating 50 grams of a specific food
• Glycemic load is based on eating a standard serving size of a specific food

Glycemic load is calculated by multiplying the GI value by the number of carbohydrates (in grams) per serving, then dividing that number by 100.

For example, an apple has a GI of 40 and contains 15 grams of carbs. (40 x 15)/100 = 6, so the glycemic load of an apple is 6.

Glycemic Load Values

In theory, foods with a low GI would also have a low GL, but that isn't always the case. Research from the International Carbohydrate Quality Consortium (ICQC) suggests that glycemic load is a more reliable indicator of how a particular carbohydrate affects blood sugar.

Like GI values, GL values can also be broken down into three ranges:

• Low GL: 10 or less
• Medium GL: 11 to 19
• High GL: 20 or more

Some foods fall under the same category for both glycemic index and glycemic load. For example, apples and oranges are both low GI and low GL, while cornflakes and boiled potatoes have both high GI and high GL. 

But for other foods, the glycemic index and glycemic load are different. For example, bananas have a low GI but a medium GL and dates have a low GI and a high GL.

A food that perhaps best highlights the difference between glycemic index and glycemic load is spaghetti. Both whole grain spaghetti and spaghetti made from white flour are considered low GI (48 and 49, respectively). However, whole wheat spaghetti has a medium GL (14) while regular, white flour spaghetti has a high GL (20). 

Glycemic Load Chart for Common Foods

The following charts highlight low, medium, and high GL foods based on data from the Linus Pauling Institute at Oregon State University. 

The Best Way to Test a Food's Impact

The American Diabetes Association states that carbohydrate amount (grams of carbohydrates) and available insulin may be the most important factors influencing blood sugar response after eating and should be considered when developing an eating plan. 

The most reliable way to assess how your body is affected by certain foods is to test your blood sugar two hours after a meal or use a continuous glucose monitoring system.

For most people, an ideal blood sugar result is less than 180mg/dL two hours after the start of a meal. If you are not sure of what your target blood sugar should be, discuss it with your physician.

A Word From Verywell

Paying attention to the glycemic index of foods can be a useful method to help avoid sudden spikes in blood sugar. However, the glycemic index is not the be-all, end-all of blood sugar control, and it shouldn’t be your only tool for managing your blood sugar. You might try using the glycemic index along with lifestyle changes, like exercise and eating balanced meals, to find what works best for you.

— ALLISON HERRIES, RDN, MEDICAL EXPERT BOARD